This application claims the priority of German patent document 100 45 385.6, filed Sep. 14, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a system for the electronic control of an actuator assigned to an automatic control system in a motor vehicle.
Such a system is known, for example, in conjunction with an automatic control system for the steering angle control of the front wheels of a vehicle from German patent document DE 41 10 148 C2. An electric servo motor on the steering column, for example, is assigned to such a automatic steering angle control system as an actuator. In such systems, the maximally possible adjusting speed is always defined with regard to the control quality, particularly with regard to the stability of the control circuit, for controlling the actuator.
For example, in superposed steering by an automatic steering angle control system, the handling of the motor vehicle is changed by an electronically controlled servo motor on the steering column in order to increase the safety and the driving comfort, and to improve the interaction between the driver and the vehicle. The steering intervention of the automatic control system automatically carried out by the superposed steering in this case is equal to the manual steering intervention by the driver. Such a servo motor can implement steering interventions which correspond to a manual steering angle of 100xc2x0.
In view of this background, it is particularly important to limit the effects of a faulty intervention of the automatic control system in all conceivable fault situations. In the most unfavorable case in this context, the servo motor starts to run at a maximal adjusting speed. This may occur, for example, as a result of a faulty jump in measuring signals which represent the operating parameters or input quantities for the automatic control functions. This jump may have the effect of a jump in the actuator adjusting command of the control unit which, in turn, causes the actuator to follow the new adjusting command as fast as possible, thus at a maximal adjusting speed.
In such a case, a fault detection unit must detect the failure within a time period (fault latency period) which is sufficiently brief that a malfunction is occurring and must immediately switch off the system. The switch-off may result in various measures for the actuator or servo motor (compare FIG. 1a):
Measure A: The servo motor can no longer be moved into a switch-off position in a controlled manner. It can still be stopped only in the momentary position. In the case of a motor vehicle, the driver must be able to correct this xe2x80x9coffset steering anglexe2x80x9d as fast as possible.
Measure B: The servo motor which has xe2x80x9crun awayxe2x80x9d at the maximal adjusting speed is moved to zero (or another defined adjusting position) after the detection of a malfunction and is held there. The fault latency period (tf1) represents the essential quantity here: the longer the fault latency period, the farther the adjusting angle can move away from its desired position and the larger the direction and course deviation of the vehicle from the desired path in the example of the automatic steering angle control system. If, for example, the fault latency period amounts to 200 ms, at a maximal adjusting speed of 300xc2x0/s, a steering angle of 60xc2x0 can be reached.
In FIG. 1a, time t is shown on the abscissa and the steering angle w3 of the actuator (in the event of a faulty intervention) is illustrated on the ordinate. The area under the steering angle course in FIG. 1a is proportional to the course angle deviation which occurs as a result of the faulty adjusting intervention in the example of the automatic steering angle control system. A linear vehicle model is assumed in this case. This result reduces as much as possible the area below the steering angle course according to the faulty adjusting intervention over the time. FIG. 1b shows how, in the case of both switch-off measures A and B, a shortening of the fault latency period (here tf2 less than tf1) reduces the area. The adjusting courses from FIG. 1a are indicated in FIG. 1b by a broken line. The switch-off measure B results in a finite area which is reduced by shortening the fault latency period. In the case of switch-off measure A, the area or the course angle deviation continues to increase over the time t but at a reduced speed.
The fault latency period results from the computing time which is required by a fault detecting unit in the control unit for checking or detecting the fault. Thus, the fault latency period cannot be arbitrarily shortened. Furthermore, the fault latency period depends on the type of the faulty function. For faults in the control unit or in the actuator control, a relatively short fault latency period can, for example, be achieved. For faults in the measuring signals, for example, for the determination of the yaw rate or of the lateral acceleration, which in the case of automatic steering angle control systems are normal operating parameters or measuring quantities on which the actuator control depends, the identification of a fault may be significantly more difficult, and the fault latency period may therefore be much longer.
One possible remedy is to double the number of sensors, which are difficult to monitor, determining the plausibility of the signals by a mutual comparison. The disadvantage here are the costs of the additional sensors.
It is therefore an object of the invention to increase the reliability of an automatic control system without additional costs when faults occur.
This and other objects and advantages are achieved by the control system according to the invention, which is based on the idea of limiting the time-related derivation of the actuator adjusting command that is dependent particularly on the sensor signals which are difficult to monitor. According to the invention the actuator is operated only at a reduced adjusting speed (for example, at 70% of the maximally possible adjusting speed) already in the no-fault operation. After detection of a fault (thus after the expiration of the fault latency period tf1; compare FIG. 1c), the actuator is preferably moved back to the defined adjusting position at a maximum possible adjusting speed (measure A) or is held at the current position (measure B). As illustrated in FIG. 1c, without any shortening of the fault latency period (tf1), a clearly reduced area is therefore obtained under the adjusting course. For a comparison with an actuator control without the invention, FIG. 1c shows, in the example of the automatic steering angle control system, the adjusting courses or the courses of the steering angle w3 of the actuator of FIG. 1a by means of a broken line.
The reduction of the maximal adjusting speed in normal operation is achieved by means of an adjusting rate limiting device between the automatic control and the actuator.
An automatic control system can, for example, carry out several partial functions, one automatic control function unit being assigned to each partial function in the control unit. In this case, a first group of partial functions or of control function units may not depend on measuring signals with a long fault latency period, so that these control function units can operate the actuator at a maximally possible adjusting speed. A second group of partial function or of control function units depends on the signals which can only be monitored with a long fault latency period. In the case of these control function units or partial functions, the adjusting command is provided with an adjusting rate limitation so that the effects of a faulty input signal will not result in an excessively fast running away of the actuator or in an excessively large area under the adjusting course. The reduction of the maximal adjusting speed in the second group of partial functions or of the control function units by means of the adjusting rate limitation is limited such that the connected power losses of this partial function do not interfere.
If the implementation of the adjusting rate limitation should result in stability problems because of closed control circuits, countermeasures can be taken which are suggested, for example, in German Patent Document DE 10021856 which has not yet been published.
The main advantage of the invention is that, in a fault detection unit with a defined minimal fault latency period and a defined upper limit for the effects of a fault event, first, no changes in the fault detection unit are required while maintaining the feasible fault latency period; second, only a minor change is required in the concerned partial functions by inserting an adjusting rate limitation or an xe2x80x9cintelligentxe2x80x9d adjusting rate limiting device; and third, doubling of the sensors or a high-expenditure further development of the algorithm for the fault detection unit or other high-cost alternative measures can be avoided.